The frequency division multiplex communication system of the present invention is of the same general type as the systems described in U.S. Pat. Nos. 3,809,815; 3,809,816 and 4,107,471.
For example, a full duplex frequency division multiplex communication system is disclosed in U.S. Pat. No. 3,809,815, in which a digital phase-locked loop synthesizer is used for generating a series of frequencies for a single sideband communication system. Single sideband transmission and reception, and frequency division multiplexing are used in the system described in the patent to provide channelization. A bandpass filter provides the necessary inversion of transmitting and receiving frequencies required to establish full duplex communication.
U.S. Pat. No. 3,809,816 discloses an appropriate system for generating a frequency pattern to provide transmitting and receiving frequencies inversion in a frequency division multiplex communication system. The relationship between the transmitter intermediate frequency and the frequency transmission band required for proper operation of the system is also described in the patent.
U.S. Pat. No. 4,107,471 discloses a frequency division multiplex communication system in which one of the sidebands of a modulated intermediate frequency carrier is used for communication and the other sideband is used for other purposes. In the system described in this patent, inversion of the transmitting and receiving frequencies is achieved by using a bandpass filter having a passband of twice the frequency difference between the transmitter and receiver intermediate frequency, rather than the passband used in the systems of the previous patents of twice the transmitter intermediate frequency. In the system of U.S. Pat. No. 4,107,471, both sidebands are independently available with either the upper or lower sideband being assigned for carrying the transmitted signal and the other sideband being used for other purposes.
Briefly stated, U.S. Pat. No. 4,107,471 is directed to a frequency division multiplex full duplex communication system using a first intermediate frequency carrier for transmission and a different intermediate frequency carrier for reception; in conjunction with a sideband filter technique to achieve a desired frequency in a pair of communication transceivers.
The system of the present invention, on the other hand, causes sideband filters to be switched about a fixed intermediate frequency for transmission and reception, in conjunction with a network frequency translator which separates the frequency bands for transceiver transmission and reception. The use of the two sidebands of a single fixed intermediate frequency carrier in the system of the present invention for transmission and reception provides for effective use of the frequency spectrum. The switchable feature of the sideband filters facilitates the communication between any two transceivers in the system. The use of a network frequency converter or translator separates the transceiver transmission and reception frequencies into two distinct non-overlapping frequency bands. As a result, echo interference is significantly minimized even though the transmission and reception carriers share a common intermediate frequency.
The system described in U.S. Pat. No. 4,107,471 is different from the system of the present invention in that two intermediate frequencies are used, as noted above, and a total of four sidebands, two for each intermediate carrier frequency, are available for two-way information communication and out-of-band signalling. In the system of the present invention, the use of one common intermediate frequency with the associated two sidebands being available respectively for transmission and reception is most efficient in frequency spectrum utilization, as compared with the four sidebands required in the prior system of U.S. Pat. No. 4,107,471.
The system of the present invention in the embodiment to be described is made up of a number of user stations, with each user station including a transceiver. A common communication channel, such as a two-directional coaxial cable, or a wireless link, is provided and each transceiver is coupled to the common channel. Each transceiver includes a first transmission modulator which provides a first step single sideband amplitude modulation of an intermediate frequency carrier by the information signals, and a second transmission modulator which provides a second step modulation function which shifts the frequency of the single sideband amplitude modulated intermediate frequency carrier by a predetermined amount according to the specific frequency channel assignment for the particular user station. Each transceiver also includes a reception demodulator which, as a first step, converts a received single sideband amplitude modulated signal into the selected intermediate frequency range, and then as a second step demodulates the converted signal.
A common network frequency converter is also coupled to the communication channel, and it serves to shift the transmission frequency band of the individual transceivers into a different non-overlapping reception frequency band, and to retransmit the shifted frequency band to the transceivers.
The amount of frequency conversion provided by the reception demodulator in an individual transceiver is a combination of the network conversion frequency from the frequency converter and the frequency of the channel assigned to the particular transceiver.
The intermediate frequency used for single sideband demodulation in any user station transceiver is the same as the intermediate frequency used in that user station transceiver for transmission modulation.
Upper and lower sideband filters are provided in each transceiver, as mentioned above, one of the filters being used for transmission and the other for reception. When two transceivers establish a communication link with one another, the sideband filter assignment for one of the transceivers for transmission and reception is made to be opposite to the sideband filter assignment for the other transceiver for transmission and reception. In this way, the information signal transmitted by upper sideband modulation in the first transceiver will be demodulated by an upper sideband demodulation in the second transceiver, and the second transceiver will transmit its information signal with a lower sideband modulation which is compatible with the receiving sideband filter of the first transceiver. Sideband filter switching means are provided in each transceiver in order that any two transceivers in the network may form a pair of communicating transceivers, with the format described above.